objectives of this research are to improve host immunocompetence after T cell depletion and to induce T cell responses toward tumor-specific antigens in the setting of minimal residual neoplastic disease. There are three primary pathways by which new T cells may be regenerated: thymic-dependent generation from marrow-derived progenitors, extrathymic generation from prethymic progenitors, and peripheral expansion of mature T cells. Using murine models, we have shown previously that thymic-dependent pathway predominates in hosts with robust thymic function, whereas thymectomized hosts regenerate T cells primarily via peripheral expansion of residual mature T cells. Follow-up studies in patients recovering from intensive chemotherapy provided evidence that thymic-dependent T cell regeneration in humans is subject to an age related decline which occurs relatively early in life. More recent studies in adults after moderately depleting chemotherapy revealed inefficient rises in CD4+ T cells bearing a memory phenotype with a high rate of apoptosis in cells undergoing peripheral expansion. Therefore, results thus far suggest that peripheral expansion is a central pathway for T cell regeneration in humans in states of thymic insufficiency, but that it is limited in its capacity to restore host immune competence. Subsequent studies have focused upon immune reconstitution in mice and humans with suboptimal thymic function. In an effort to study potential etiologies of age- and disease- or therapy-associated thymic involution, T cell regeneration was studied in aged mice after lethal irradiation and infusion of young bone marrow (BM). The results show that age associated thymic involution is not reversed by the administration of young BM, suggesting that age related changes within the thymus (rather than the BM), are primarily responsible for age associated thymic insufficiency. These studies also provided the important observation that when monitored carefully, aged thymi were able to regenerate sizable numbers of thymic-derived progeny over time. These results suggest that even aged thymi, over a prolonged timeperiod, may be capable of contributing significantly to T cell regeneration. The role of inadequate marrow function as a potential cause for diminished thymic regenerative capacity after intensive chemotherapy was further investigated in a clinical study of immune reconstitution after autologous peripheral blood progenitor cell transplantation. In this study, patients received large doses of CD34+ progenitors which rapidly restored trilineage hematopoiesis. Investigation of immune reconstitution in this population however revealed prolonged profound reductions in CD4+ T cell number which was not restored despite infusion of large numbers of pluripotent CD34+ hematopoietic progenitors. This study corroborates the results in the aged murine model described above, by providing evidence that thymic function remains inadequate despite treatment with substantial numbers of pluripotent hematopoietic progenitors. In order to better analyze the ability of low level thymic function to diversify the TCR repertoire over time in humans, current work is focused upon evaluation of TCR repertoire diversity in a series of pediatric patients treated with highly active antiretroviral therapy for HIV infection. We have hypothesized that even suboptimal thymopoiesis in this patient population may lead to diversification of the TCR repertoire over time. In an attempt to measure such changes in TCR diversity, we have developed a semi-quantitative approach for the measurement of TCR repertoire diversity using CDR3 size analysis of serially diluted purified CD4+ T cells. Preliminary results using this approach suggest that significant reductions in TCR diversity exist in children with HIV infection, and that these these changes are not rapidly reversed restored despite substantial increases in CD4+ T cell numbers with HAART. Further studies in mice have attempted to identify the requirements for successful T cell response induction in T cell depleted hosts. Using a model of HY skin graft rejection, after T cell depletion, we have shown that thymectomized mice which have undergone immune reconstitution via peripheral expansion are unable to reject HY disparate skin grafts while hosts reconstituted via thymic-dependent pathways reject grafts normally. Rejection is accomplished in thymectomized mice if sufficient numbers of T cells are provided which has been shown to represent approximately 10% of the normal T cell repertoire. In these experiments, we have also shown that IL-7 potently enhances thymic-independent T cell regeneration and that IL-7 therapy restores the capacity to reject HY disparate grafts in thymectomized hosts reconstituted with insufficient T cell numbers. Mechanistically, this appears to be related, at least in part, to IL-7's ability to inhibit programmed cell death since inhibition of PCD by the use of bcl-2 transgenic mice also enhances immune competence in this model. Therefore, these results suggest that PCD may serve to limit host immune competence during peripheral expansion and raise the possibility that modulation of PCD in vivo may serve as a useful approach for enhancing immune competence in thymic-deficient TCD hosts. The basic studies described above which have elucidated the propensity for skewing of regenerated T cell populations in thymic-deficient hosts also suggest that such skewing could be exploited in the context of clinical approaches to induce responses to nominal antigen during a period of immune reconstitution. We have developed a clinical trial to test this hypothesis wherein T cell populations are collected prior to intensive chemotherapy in patients with Ewing's sarcoma and alveolar rhabdomyosarcoma. Upon completion of cytotoxic therapy, immune reconstitution/immunotherapy is undertaken which consists of reinfused T cells, tumor-specific peptide pulsed antigen presenting cells and interleukin-2.